1. Field of the Invention
The present invention concerns a suspension unit for vehicles with built-in damper.
To be more precise, the present invention concerns a suspension unit of the type comprising:
a support adapted to be attached to a framework of the vehicle,
a longitudinal arm,
means coupling a rolling member to said arm defining a transverse first axis of rotation of said rolling member relative to said arm,
first means coupling said arm to said support defining a transverse second axis of rotation of said arm relative to said support, said second axis being parallel to said first axis and offset from it longitudinally,
second means coupling said arm to said support defining an angular equilibrium position of said arm for rotation about said second axis relative to said support and elastically enabling rotation of said arm about said second axis relative to said support in two opposite directions from said angular equilibrium position, and
means damping said rotation of said arm.
2. Description of the Prior Art
In known suspension unit of this type the damper means are of the hydraulic type and employ two hydraulic fluid chambers inside the arm the volumes of which vary concommitantly, inversely to each other, when the arm pivots about the second axis relative to the support; the two chambers are connected by means defining a restriction and which permit the fluid to flow from one chamber to the other sufficiently slowly to damp rotation of the arm.
The circulation of the hydraulic fluid, and in particular that through the restriction means, naturally heats the fluid, proportionately to the degree to which the suspension is loaded, that is to say more strongly as the arm executes faster and more frequent rotation movements relative to the support; this heating entails provision of means for cooling the hydraulic fluid, either integrated into the arm, which results in a considerable increase in its overall dimensions, weight and inertia and thus compromises the quality of the suspension, or separately from the arm, which makes it necessary to provide flexible connecting hoses between the arm and the means cooling the hydraulic fluid which are particularly vulnerable and are particularly inconvenient since suspension units of the type indicated in the preamble are generally fitted to civilian or military vehicles designed for use on unusually difficult terrain, where there is a high risk of damaging such hoses.
It also appears that such damper means using circulation of hydraulic fluid through restriction means cannot withstand rapidly applied loads, unless of complex and costly design.
To be more precise, it is generally accepted that damper means are satisfactory when they apply a damping force (or moment) that is:
appproximately proportional to the speed of vertical displacement of the rolling member relative to the vehicle (or the speed or rotation of the arm relative to the support) provided that this speed remains below a predetermined threshold, and
approximately constant immediately the speed exceeds this predetermined threshold.
Hydraulic fluid damper means as provided until now in oscillating arm suspension units only meet this criterion up to a second threshold of the speed of vertical displacement of the rolling member relative to the vehicle is reached; immediately the second threshold is exceeded, the damping force or moment is once again proportional to the speed of vertical displacement on the rolling member relative to the vehicle and may reach values likely to result in damage to or even destruction of the hydraulic circuit and the hydraulic fluid constituting the damper means.
In an attempt to alleviate these disadvantages, it is proposed in U.S. Pat. No. 4,447,073 to implement damper means for a suspension unit of the type indicated hereinabove in a form comprising:
two complementary friction brake members,
means immobilizing said members against rotation about the second axis respectively relative to the arm and relative to the support,
means pressing together the friction brake members, and
means conditioning the force with which the friction brake members are pressed together according to the speed of rotation of the arm about the second axis relative to the support in one direction only referred to as the "first direction".
In practise these conditioning means consist of a single-acting piston pump cooperating with a cam to alternately draw up hydraulic fluid in a storage tank and discharge it to hydraulic jack means constituting the means pressing together the complementary friction brake members, as the arm rotates, with the result that:
on rotation of the arm in the first direction, corresponding to a discharge phase, the value of the force with which the friction brake members are pressed together, that is to say the damping force or the moment of this force about the second axis, increases from a minimum to a maximum and then decreases in accordance with a predetermined law, and
on rotation of the arm in the second direction, corresponding to an aspiration phase, the value of this force, that is of the damping force or moment, remains substantially at the aforementioned minimum value.
The hydraulic fluid then no longer contributes to the damping proper, as the heating phenomena associated with the damping action are then observed at the level of the complementary friction brake means, that are less vulnerable and easier to cool, and the value of the damping force or moment then remains limited, but the way in which this force or moment changes and the structure of the damper means disclosed in the aforementioned document, which are indissociably associated with each other, cannot be regarded as totally satisfactory since, in particular, it is not possible to exert any influence over the damping action on rotation of the arm in the second direction.
An object of the present invention is to alleviate these disadvantages.